Assuming you're using bash or similar, this is built into the default completion options which get installed with fzf: https://github.com/junegunn/fzf#fuzzy-completion-for-bash-and-zsh

tldr; enter start file or directory, append ** and hit Tab. So if you'd enter cd /foo/** then tab opens fzf with /foo as start directory.

edit at the time of writing the commands for which this works are hardcoded in fzf's bash helpers, which is why this works for cat and cd but not for tac or nano. This is the complete list:

awk cat diff diff3
emacs emacsclient ex file ftp g++ gcc gvim head hg java
javac ld less more mvim nvim patch perl python ruby
sed sftp sort source tail tee uniq vi view vim wc xdg-open
basename bunzip2 bzip2 chmod chown curl cp dirname du
find git grep gunzip gzip hg jar
ln ls mv open rm rsync scp
svn tar unzip zip

To add other commands use this in e.g. your .bashrc, after the place where fzf gets sourced (something like [ -f ~/.fzf.bash ] && source ~/.fzf.bash):

__fzf_defc "tac" _fzf_path_completion "-o default -o bashdefault"

Alternatively: open an issue to ask for the commands you want to be added to fzf by default, things like tac and nano are super common anyway.